Good detective work.Okay, I have gotten further with this.
The VPN works fine on our current Ubiquiti setup, with FTTC --> Draytek Vigor 130 bridged --> Ubiquiti USG --> Unifi Wi-Fi
The VPN works fine through 5G --> ZTE router in standard router mode --> Wi-Fi.
The VPN does has a problem with 5G --> ZTE Bridged mode --> Ubiquiti USG --> Unifi Wi-Fi
The ZTE router in bridge mode is the weak link.
Might just buy the TP-Link Archer NX210 and give that a go.
Yeah I think it's £199 at the moment.